Annual precipitation, which varies widely throughout the type because of differences in elevation, topography, and geography, ranges from 250 mm (10 in) at low elevations where the type adjoins the desert or grassland vegetation to 560 mm (22 in) or higher at the upper reaches (62,67,73). Locally, amounts as high as 690 mm (27 in) have been recorded, as along the Mogollon Rim in northern Arizona (15).
Seasonal distribution, which also varies considerably, is related to prevailing storm patterns. In eastern New Mexico, for example, approximately 75 percent of the annual precipitation occurs during the warm season (April through September) from storms originating in the Gulf of Mexico, whereas the percentage decreases as these summer storms lose intensity during their northwesterly movement (62). Nevertheless, summer precipitation throughout much of northern Arizona and the south-central and eastern portions of Utah is still about equal to, or slightly greater than, winter moisture (14,42). Furthermore, as much as one-third of the rainfall may occur during July and August (67). In contrast, woodlands of Nevada and northern Utah receive more precipitation during the cool season (October through March), primarily from Pacific winter and spring storms (13,14). Snow depths are not great, except at higher elevations and more northerly latitudes, but even then, melt generally occurs within a few days, especially on south-facing slopes (62).
The mean annual temperature in pinyon-juniper woodlands varies from 4° to 16° C (40° to 61° F); extremes may fall to -35° C (-31° F) and reach 44° C (112° F). January means may be as low as -10° C (14° F) in the more northerly portion of the type, and about 6' C (430 F) near the southern limits. Mean July temperatures are less variable, ranging from 20° to 27° C (68° to 81° F). The frost-free period ranges from about 90 to 205 days, the shorter period typifying more northerly latitudes and higher elevations (59,67,73).
Because of wide variation in temperature and the amount and distribution of precipitation, the following classification has been proposed to better characterize the climate of pinyon-juniper woodlands in Arizona and New Mexico (62):
Precipitation Climate Winter Summer mm Cool, moist 230 to 280 180 to 230 Warm, moist 250 to 330 150 to 230 Cool, winter dry 130 to 180 180 to 230 Warm, winter dry 100 to 180 200 to 280 Cold, winter dry 100 to 150 200 to 250 Cold, summer dry 180 to 230 100 to 150 Warm, summer dry 180 to 230 100 to 150 in Cool, moist 9 to 11 7 to 9 Warm, moist 10 to 13 6 to 9 Cool, winter dry 5 to 7 7 to 9 Warm, winter dry 4 to 7 8 to 11 Cold, winter dry 4 to 6 8 to 10 Cold, summer dry 7 to 9 4 to 6 Warm, summer dry 7 to 9 4 to 6
- Burns, Russell M., and Barbara H. Honkala, technical coordinators. 1990. Silvics of North America: 1. Conifers; 2. Hardwoods. Agriculture Handbook 654 (Supersedes Agriculture Handbook 271,Silvics of Forest Trees of the United States, 1965). U.S. Department of Agriculture, Forest Service, Washington, DC. vol.2, 877 pp. http://www.na.fs.fed.us/spfo/pubs/silvics_manual/table_of_contents.htm
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